1. Field of the Invention
The present invention relates to a device for measuring the tensile force on a thread. It relates more particularly to a device in which a magnet on a spring plate is displaced by the thread tension, and the changing magnetic field of the magnet is sensed by a fixed Hall sensor which produces an output signal indicative of the thread tension.
2. Description of Related Art
A number of thread tension measuring devices are known. If they are not of purely mechanical construction, they operate electrically by means of inductive or capacitive displacement transducers, as well as wire strain gauges. In order to obtain a useful indication from such a device, an amplifier is necessary. Thus, for instance, in Federal Republic of German Pat. No. 26 44 358 there are disclosed two thread tension sensors which operate with a pair of electric induction coils and a differential amplifier. In order to obtain an indication which is at all useful with these described devices, expensive and costly amplifiers are necessary. The operation of these devices can be greatly impaired by accumulations of dirt which may form between the induction coils as well as in the differential amplifier. The dirt narrows the displacements to be measured, and thus the device becomes ineffective. Furthermore, the devices require fine adjustments to a tension spring and an adjustment screw which are complicated and imprecise and may lead to erroneous output values.
In a further Federal Republic of Germany patent application, No. AS 24 34 158, a device is described which operates by means of an electromagnet. In this device also, an amplifier must be provided in order to obtain a usuable display, which is expensive and is scarcely feasible from an economic standpoint.
Another disadvantage of the latter device lies in the support arrangement of the feeler arm. The device must operate properly at thread tension of about 2 to 3 g. Nevertheless, this device includes a pivot point, on which the feeler arm pivot, that may exhibit a high moment of friction. This arrangement is very trouble-prone, since the slightest amount of dirt or debris--and this is inevitable, for instance, in texturing--impedes the motion of the pivot point and makes the device ineffective.